Anti-reflection coating is provided to reduce the reflection that occurs from the difference in refractive index between the media of optical elements that are incorporated in an optical system. If such reflected light rays are allowed to reach the image plane, then a ghost or a flare can appear, severely degrading the optical performance of the system. In recent years, optical systems have been required of increasingly higher optical performance, so the anti-reflection coating provided on optical elements in an optical system is likewise required to provide increasingly lower degrees of reflection in a range of incident angles wider than ever before.
To meet such demands, progress has been made in the field of multilayer-film design technique, where various materials and film thicknesses are applied in combination, as well as in the field of multilayer coating technique (for example, refer to Japanese Laid-Open Patent Publication No. 2000-356704).
However, there has been a problem in the prior-art anti-reflection coating. If the angle of light rays incident to an optical surface where anti-reflection coating is provided increases greatly, the light rays entering obliquely, then this condition causes a rapid change in the property of the anti-reflection coating and abruptly weakens the effectiveness of the anti-reflection coating. As a result, there is an increase in the amount of light reflected. In a case where such a reflective surface exists singularly in an optical system, the reflected light is directed toward an object being observed through the system, so the above-mentioned problem does not affect directly the optical performance of the system. However, if such surfaces exist in plurality, then the system can experience at high probability a ghost or a flare, which is caused by the reflected light reaching the image. Nowadays, lenses are fabricated to have large diameters, and this manufacturing trend contributes to increases in the angular range of light rays that are incident to optical elements. As a result, optical systems are now prone to experience ghosts and flares.